How can you get rights for the 2nd admin-user in Ubuntu?
My window-manager is Ion3.
I get the following errors for X11 applications run by the second admin-user.
masi@masi-desktop:/home/noa$ firefox
No protocol specified
Error: cannot open display: :0.0
This suggests me that you need to have rights to the display.
However, su -c xterm is not working, since the root password is disabled in Ubuntu.
Try sudo; however you should not need to be root to get the display. I'm unaware of Ion3, but I'm sure more errors will crop up after you get root, in which case you can update your question.
The problem can be solved by giving every process an access the display by
sudo xhost +
However, this can be a security threat, since any process in your network can then access your Screen and possibly too other input devices such as mouse.